1. Field of the Invention
The invention refers in general to an electrical heating system and to a control unit for a motor vehicle heating system. In particular, the invention refers to those electrical heating systems which have PTC-Elements and which are used to heat the passenger compartment of a motor vehicle.
2. Description of the Related Art
For the application in motor vehicles, Electrical Heating Systems are used to heat the passenger compartment of the vehicle and its engine—in particular in those vehicles with combustion engines that have been optimised for low fuel consumption. Such heating systems are also suitable in other application fields, for example, in room climate control, within industrial systems and in household appliances and such like.
The increased efficiency of combustion engines, which have been optimised for low fuel consumption, leads to a considerable reduction in the amount of heat generated within the vehicles. Consequentially, on journeys where the engine is only partially loaded, a very much reduced heating of the vehicle interior is obtained. To redress the balance in this deficit, auxiliary electrical heaters are utilised, in particular in vehicle air conditioning systems.
Such motor vehicle heating systems consist of radiator elements which are made up of PTC heating elements, which are mounted within a frame and secured by means of spring elements. PTC elements are temperature dependent semiconductor resistances whose electrical resistance increases as the temperature rises. When an electrical potential is applied across such PTC elements, an electrical current flows through them, the amount of which is dependent on their resistance. The PTC elements are thus heated up to a predetermined temperature. The heat that is so generated by these PTC elements is then delivered into a flowing stream of air or water.
Because of their inherent characteristic of being able to increase their electrical resistance exponentially when the temperature increases above a characteristic value, PTC elements can be utilised as self regulating heating elements, since, when the characteristic temperature is attained, they automatically, independently, regulate themselves back. By means of this self regulatory process of the PTC elements, the consumed electrical energy corresponds exactly to the delivered thermal energy.
A fault may occur, however, when the air or water flow is interrupted, particularly when the fan generating the air flow, or the pump providing the water current, fails. In such a case, the temperature of the PTC heating element increases up to the maximum PTC temperature. As a result of this, the electrical power is reduced to a value which is only a fraction of the maximal possible power.
A fault can also occur when the control of the electrical heating system reduces the input air flow significantly or interrupts it completely without reducing the corresponding amount of heat that is to be generated. Such a situation can occur because of a fault within the automatic heating controller and/or when the heating system is manually regulated (misuse).
This temperature dependence of the electrical power Pel, which is converted into heat by the PTC element, is shown graphically in FIG. 2. plotted against the temperature of the PTC element. For a given voltage supply, a working point is established within the PTC element which is dependent on the ambient temperature, that is, the air temperature, and the heat conducted from the PTC element to the medium that is to be warmed. Such a working point A1, is graphically displayed in FIG. 2 for the temperature T1.
Should a fault in the input air flow occur, the PTC element will become warmer because the generated heat cannot be dissipated. The temperature of the PTC element therefore rises. Its working point shifts further downwards along the plotted curve as shown by the new working point A2 in FIG. 2 corresponding to the temperature T2. Thereby, the amount of generated heat and the corresponding amount of electrical power consumed is reduced.
In such a case, the residual power consumption is between 6% and 8% of the maximum rated power value; for an electrical heating system rated at 1 kW maximum, such residual power consumption still lies between 60 watts and 80 watts.
Even when the interior heating of a motor vehicle fails because of a breakdown of the fan, electrical power is nevertheless consumed. As a consequence, the electrical heating system, especially the radiator elements, can heat up to a much higher temperature than is encountered in normal operation. This temperature, which lies significantly above normal temperatures, can lead to consequential damage to components in the vicinity, for example, the melting of plastic fasteners.